Site Navigation

Site Mobile Navigation

In Update on Sperm, Data Show No Decline

It is one of the most fraught topics in environmental health. Are men becoming less fertile, with declining sperm counts and diminishing sperm quality? If they are, then sperm might be an early warning sign of environmental dangers. And the prime suspects have been substances like plastics and pesticides that can have weak estrogenlike effects on cells.

But now 15 years of data from 18-year-old Danish men taking their military physicals show no decline in sperm counts, after all. The idea that sperm counts were plummeting began with an alarming paper published in 1992 by a group of Danish researchers. Sperm counts, they reported, declined by 50 percent worldwide from 1938 to 1991, and the trend would continue, they said.

Many other researchers criticized the data’s quality, citing flaws like a lack of standardized methods of collecting semen, methodological issues in semen analysis, biases in the ways men were selected, and variations in the length of time men abstained from ejaculating before their semen was collected.

The study, said Dolores Lamb, a fertility expert at Baylor College of Medicine and president-elect of the American Society of Reproductive Medicine, “was problematic and raised alarms in society without critical thinking about the caveats and weaknesses inherent in the data and its analysis.”

Nonetheless, the paper was highly influential. It was cited by 1,000 subsequent scientific papers.

Other researchers soon published their own studies, but methodological problems persisted. And the later studies came to contradictory conclusions, with some saying sperm counts were declining and others saying they were not. The result was a body of evidence so poor that a panel of experts assembled by the National Academy of Sciences in 1999 said its members could not come to a consensus on whether counts were declining, mostly because of seriously flawed studies.

Meanwhile, the same Danish group that got the debate started began a study that analyzed annual semen samples collected from 18-year-old men who were being examined for their fitness for the military — a requirement in Denmark. Over the past 15 years, a total of 5,000 men provided semen for analysis.

That design was an improvement over older studies, Dr. Lamb said. The data are from men of the same age and from one geographic area (sperm numbers and quality can vary from one region to another). Analysis of sperm is better now than it was in years past. And with 15 years of data, she said, any decline in sperm numbers or quality should have been evident.

The problem was that the group did not publish its data, even though, said Dr. Jens Peter Bonde, a fertility researcher at Copenhagen University Hospital, “we have asked for these findings — they are of great public interest.” Dr. Niels Erik Skakkebaek of the University of Copenhagen, who initiated the study, said he wouldn’t comment on the data before the research appears in a scientific journal. And he would not say when that might be.

But the data have been published anyway, in an unusual manner.

In a telephone interview, Dr. Skakkebaek said the research group’s current leader, Niels Jorgensen, sent the data to the Danish Ministry of Health, which helped pay for the study, and the ministry then posted the data on its Web site. Dr. Skakkebaek was angry, saying in an e-mail: “The trend data has not been appropriately scientifically scrutinized. Also, I cannot guarantee that the civil servant in the ministry put our data into the figure without mistakes.”

Now, an American journal, Epidemiology, has published the data in a commentary and discussed them in an editorial.

The commentary, by Dr. Bonde, includes a graph of the data and says they constitute “the best longitudinal semen data yet available.”

The journal’s editor, Dr. Allen Wilcox, said he decided to reproduce the figure from the ministry Web site because the data are so important. Yet, he wrote in the editorial, “the presentation of a few raw data on a Web site — or in a commentary — is hardly the preferred way to advance science.” But, he added, “neither is it acceptable for valuable data to be held in storage.”

A version of this article appears in print on June 7, 2011, on page D5 of the New York edition with the headline: In Update On Sperm, Data Show No Decline. Order Reprints|Today's Paper|Subscribe